Papermaking is a well known art. In papermaking cellulosic fibers and a liquid carrier are mixed together. The liquid carrier is drained away and the resulting embryonic web of cellulosic fibers is dried.
Drying is typically accomplished in one of two manners, through air drying or conventional drying. Through air drying relies upon blowing hot air through the embryonic web. Conventional drying relies upon a press felt to remove water from the web by capillary action.
Through air drying yields paper having regions of different densities. This type of paper has been used in commercially successful products, such as Bounty paper towels and Charmin and Charmin Ultra brands of bath tissues. However, there are or may be situations where one does not wish to utilize through air drying.
In these situations, conventional felt drying is used. However, conventional felt drying does not necessarily produce the structured paper and its attendant advantages. Accordingly, it has been desired to produce structured paper using conventional felt drying. This has been accomplished utilizing a conventional felt having a patterned framework thereon for imprinting the embryonic web. Examples of these attempts in the art include commonly assigned U.S. Pat. Nos. 5,556,509, issued Sep. 17, 1996 to Trokhan et al.; 5,580,423, issued Dec. 3, 1996 to Ampulski et al.; 5,609,725, issued Mar. 11, 1997 to Phan; 5,629,052, issued May 13, 1997 to Trokhan et al.; 5,637,194, issued Jun. 10, 1997 to Ampulski et al.; 5,674,663, issued Oct. 7, 1997 to McFarland et al.; and 5,709,775 issued Jan. 20, 1998 to Trokhan et al., the disclosures of which are incorporated herein by reference.
There are occasions where a conventional felt is used without a patterned framework thereon. In such cases, a paper web may be transported on a separate imprinting fabric and compressed in a compression nip formed between two rolls.
U.S. Pat. No. 4,421,600 issued Dec. 20, 1983 to Hostetler discloses an apparatus having two felts, three pressing operations, and a separate woven imprinting fabric. In Hostetler the web is transported on the imprinting fabric through the pressing operations before being delivered to the Yankee dryer.
Another such attempt in the art is illustrated by U.S. Pat. No. 4,309,246 issued Jan. 5, 1982 to Hulit et al. Hulit et al. describes three configurations where a nip is formed between two rolls. In each configuration, a paper web is carried on an imprinting fabric having compaction elements defined by knuckles formed at warp and weft crossover points. The imprinting fabric, web and a felt are compressed between the rolls. The web is carried from the nip on the imprinting fabric. In two embodiments, Hulit then transfers the web from the imprinting fabric to a Yankee drying drum. In the third embodiment, Hulit does not use a Yankee drying drum.
The Hulit arrangements have several disadvantages. First, two sets of nips are required, a first nip to imprint the web and a second nip where the web is transferred to the Yankee drying drum, Hulit recognizes that dryer drums may be utilized instead of, or in addition to, the Yankee drying drum. However, Hulit does not minimize the expense and inconvenience of requiring two separate nips for the configurations relying upon the Yankee drying drum—as most commonly occurs in the art.
Another attempt is shown in European Patent 0 526 592 B1 granted Apr. 5, 1995 to Erikson et al. Erikson et al. discloses another two nip configuration. In the first nip, the paper is imprinted between a press roll and a lower press roll. There, Erikson et al. dewaters the paper by placing the press felt directly against the paper. This allows the press felt to deform into the areas of the imprinting fabric not supported by knuckles, reducing the differential density effects of the compaction caused by the imprinting fabric.
Erikson imprints the paper and transfers it to the Yankee at a lower press roll. The paper is transferred to the Yankee drying drum at this point. However, the second press drum again imprints the paper. The problem presented by Erikson et al. is that at its second nip the imprinting belt is never in registration with the imprinted pattern provided at the first nip. Thus, Erikson unduly compacts the paper and destroys the caliper it creates by imprinting at the first nip.
Furthermore, Erikson et al. like the aforementioned attempts in the art, still requires a complex two nip system in order to bring the imprinting fabric/paper web combination into contact with a dewatering felt. Erikson requires the press felt loop to be outboard of the imprinting fabric loop. This arrangement creates a very expensive proposition for retrofit to existing machinery, as additional space, drives, etc. are required to add the separate felt loop. The cost of installing such a separate felt loop on an existing papermaking machine can be quite significant.
Commonly assigned U.S. Pat. No. 5,637,194 issued Jun. 10, 1997 to Ampulski et al., the disclosure of which is incorporated herein by reference, discloses an alternative paper machine embodiment where a first dewatering felt is positioned adjacent a face of the imprinting member as the molded web is carried on the imprinting member from a first compression nip formed between two pressure rolls and a second dewatering felt to a second compression nip formed between a pressure roll and a Yankee drying drum. The imprinting member imprints the molded web and carries it to the Yankee drying drum. The presence of the first felt adjacent the imprinting member at the two compression nips results in additional water removal from the web prior to transfer to the Yankee drum.
While Ampulski et al. represents a significant improvement over the prior art, Ampulski et al., still requires a complex two nip system in order to bring the imprinting fabric/paper web combination into contact with the dewatering felts. Ampulski requires the press felt loop to be outboard of the imprinting fabric loop. This arrangement creates a very expensive proposition for retrofit to existing machinery, as additional space, drives, etc. are required to add the separate felt loop. As mentioned previously, the cost of installing such an arrangement can be quite significant.
Accordingly, the present invention provides a web patterning apparatus suitable for making structured paper on a conventional papermaking machine. The invention further provides a web patterning apparatus capable of dewatering a paper web using conventional dewatering techniques such as a suction vacuum roll.